While native p2l-ras is an essential growth-regulating component of eukaryotic cells, single point mutations are sufficient to convert the cellular form into a tumor promoting oncoprotein. In a high percentage of human tumors, one of the three endogenous ras genes is activated by a somatic point mutation. Activating mutations in principle can interfere with both regulating mechanisms: either nucleotide exchange is enhanced or GTP-hydrolysis is drastically slowed down. Over the past few years, X-ray diffraction studies on p2l have contributed greatly to the understanding of the structure and mechanism of action of this small GTP-binding protein. The structure of the GppNHp complex of p2l-ras has been refined at 1.35 A resolution. Preliminary experiments performed at the Photon Factory synchrotron, Tsukuba, Japan, indicate that diffraction data might be collected to better than I A resolution, there was a weak signal to 0.9 A and plots of R-syms and R-cryst were smooth to a resolution of 1. 1 A. The data were collected under non-optimal conditions and the crystals were not frozen, so radiation damage was a problem and our analysis had to be done on incomplete data sets. In the meantime, preliminary flash-freezing tests have been done showing that the crystals can be frozen without loosing resolution as far as can be assessed on a rotating anode source. It is helpful that the crystals are actually grown in a cryo-protectant solution. We will aim to measure diffraction the diffraction pattern of such a crystal to the limit of resolution hopefully well below I A. Data collection on BioCARS Station 14-BM-D.